Oak Mast Production and Animal Impacts on Acorn Survival in the Central Hardwoods

176 OAK MAST PRODUCTION AND ANIMAL IMPACTS ON ACORN SURVIVAL IN THE CENTRAL HARDWOODS Kenneth F. Kellner, Jeffery K. Riegel, Nathanael I. Lichti, and Robert K. Swihart1 Abstract.—As part of the Hardwood Ecosystem Experiment we measured mast production in white (Quercus alba) and black (Q. velutina) oak, and quantified the impacts of seed predators on acorn survival over a 3-year period. Specifically, we measured the proportion of acorns of each species infested with weevils (Curculio spp.), and the probability of acorn removal by seed predators from a system of semipermeable exclosures. The 3 years of the study included 2 years of high mast production in both species (2006-07) and 1 year of mast crop failure (2008). Across all 3 years, 19 percent of acorns were infested. The rate of weevil infestation was slightly higher for black oak than for white oak in each year, but infestation peaked for both species during the year of mast failure (2008). Overall, 39 percent of acorns in the exclosures were removed by seed predators. The probability of acorn removal was lower when squirrels were excluded, providing support for additive effects of different seed predators. The probability of removal was highest in 2008 during the year of mast failure. In the future, these pre- harvest data will be compared to data obtained following timber harvests conducted in winter 2009. INTRODUCTION 2001, Johnson et al. 2002). Among the most important Oak (Quercus L.) is a dominant overstory species functions of oak in the Central Hardwoods is the group in the Central Hardwoods Region. Oaks have production of hard mast, an important food source been labeled as both a keystone and “foundation” for at least 44 species of birds, small mammals, species in eastern deciduous forests (Ellison et al. and larger vertebrates such as white-tailed deer 2005, Fralish 2004), performing a wide variety (Odocoileus virginianus Zimmerman) and black bear of functions in forest ecosystems. For example, (Ursus americanus Pallus) (McShea et al. 2007). oak-dominated forests promote biodiversity in the The importance of oak as a source of hard mast has herbaceous understory because oak branch structure greatly increased in the past century due to the decline allows a large amount of sunlight to reach the forest of American chestnut (Castanea dentata [Marsh.] floor (Fralish 2004, Horn 1971). Oaks also are Borkh.) (Dalgleish and Swihart [in press], Diamond et known to provide habitat for many species of insects, al. 2000, McShea et al. 2007). fungi, and vertebrates and play an important role in hydrology and nutrient cycling (Brändle and Brandl Oaks generally are shade-intolerant species, requiring disturbance to regenerate effectively (Larsen and Johnson 1998). An active cycle of natural and/or 1 Graduate Student (KFK, NIL); Field Research Coordinator anthropogenic fire disturbance is thought to have (JKR); and Professor and Department Head (RKS), Purdue promoted oak dominance in the Central Hardwoods University, Department of Forestry and Natural Resources, before European settlement (Abrams 1992). Following 715 West State St., West Lafayette, IN 47907. KFK is corresponding author: to contact, call 765-494-9597 or settlement, cycles of land clearing for agriculture and email at [email protected]. subsequent abandonment maintained oak presence in 177 the overstory (Fralish 1997). However, the advent of 2000, McShea et al. 2007). However, some seed fire suppression and the creation of protected national predators (e.g., S. carolinensis) make many small and state parks in the 20th century have greatly caches of acorns, which may promote germination reduced the frequency of disturbance in eastern forests success (Barnett 1977, Smallwood et al. 2001, Steele (Abrams 1992, 2003). The result has been regeneration et al. 2006). failure of oaks across the Central Hardwoods Region, as well as shifts in species assemblages within the oak Regardless of animals’ influence on oak regeneration genus (Abrams 2003, Aldrich et al. 2005). This altered success, little is known about the impact of timber disturbance regime favors the establishment of shade- harvest strategies on oak mast production and tolerant climax species such as maple (Acer L.) and subsequent predation by insects and small mammals. American beech (Fagus grandifolia Ehrh.) (Fralish The Hardwood Ecosystem Experiment (HEE), a long- 2003). term replicated study in Indiana of forest ecosystem responses to timber harvest, provides an excellent Loss of oak as a canopy dominant would have experimental framework to address this knowledge important ecological and economic impacts (Johnson gap. Our objectives in this study were to compare mast et al. 2002, McShea et al. 2007). As a result, production by black (Q. velutina Lamb.) and white researchers have begun to develop forest management (Q. alba L.) oaks at several sites in southern Indiana (i.e., timber harvest) strategies to promote oak over a 3-year period, and subsequently to assess the regeneration (Dey and Parker 1996, Dey et al. 2008, impacts of acorn weevils and seed predators on the Loftis 1990). Examples of management include even- acorn crop. Following the final year of this preliminary aged methods (clearcuts and shelterwood harvests) study, the experimental sites were harvested under and uneven-aged methods (group-selection or single- several different management strategies with the goal tree selection harvests). Management efforts have of improving oak regeneration. Ultimately, our results met with mixed success; ecological variables such will serve as a baseline for identifying changes in mast as soil quality and moisture likely play a role in the production and seed predation by weevils and small outcome (Dey et al. 2009). Oak regeneration success mammals following the silvicultural treatments. is also heavily influenced by animals, including acorn weevils, small mammal seed predators, and deer Prior to the application of silvicultural treatments, (Marquis et al. 1976). we tested several hypotheses. We expected that acorn production would vary among the 3 years of the study, Weevils (Curculio L.) infest the acorns of all oak and that production would be different between the species in the Central Hardwoods. In a given year, as two oak species. Oaks are a masting species group, many as 50-90 percent of all acorns produced may be synchronizing with other trees in a region to produce infested (Gribko 1995, Lombardo and McCarthy 2008, very large or very small mast crops (Janzen 1971), but Marquis et al. 1976, Riccardi et al. 2004). Infested often lacking interspecific synchrony (Abrahamson acorns are both less likely to be dispersed (Steele et and Layne 2003, Lombardo and McCarthy 2008, Lusk al. 1996) and less likely to germinate successfully et al. 2007). We also expected that black and white (Andersson 1992, Lombardo and McCarthy 2009). oaks may be affected differently by acorn weevils; The interaction of oaks with small mammal seed Lombardo and McCarthy (2008) found that a higher predators is more complex. A large percentage of proportion of acorns in the red oak section (Lobatae) fallen acorns are eventually removed by seed predators were infested than those in the white oak section such as white-tailed deer, gray squirrels (Sciurus (Quercus), but few studies have compared infestation carolinensis Gmelin), eastern chipmunks (Tamius levels between oak species. striatus L.) and mice (Peromyscus Gloger) (McShea 178 To address the impacts of predators on the fate of Across all experimental units, 108 trees were sampled fallen seeds, we sought to isolate the individual in 2006, divided evenly among the three harvest contributions of several acorn predators on the total treatments. In 2007, an additional experimental amount of removals observed, in order to assess if unit was added in Brown County State Park; four those contributions were additive or compensatory. additional control trees were selected in this unit for We expected that gray squirrels would be most prolific a total of 112 trees sampled. Due to active timber at removing fallen acorns (Bellocq et al. 2005). In harvesting in uneven- and even-aged units in the fall of addition, we expected that less desirable acorns (i.e., 2008, 20 trees could not be sampled, yielding a sample acorns that were broken, germinated, or infested with size of 96 in the third year of the experiment. Overall, weevils) would be less likely to be taken by small black and white oaks had diameters at breast height mammals (Smallwood et al. 2001, Steele et al. 1996). (d.b.h.) of 20.0±4.4 cm (mean ± standard deviation) and 19.5±4.5 cm, respectively. STUDY AREA AND DESIGN A summary of site characteristics and the selection MATERIALS AND METHODS of research areas can be found in a previous chapter Mast Production of this report (Kalb and Mycroft, this publication). At each tree, two mast collection traps were Following the delineation of the research core areas, established. Traps consisted of a 52cm × 33cm × 32cm we selected mature black and white oak trees to plastic bin mounted atop a 2-m high polyvinyl chloride be included in the study. We chose this pair of oak pipe driven into the ground. Bins were covered with species because they are among the most dominant chicken wire to prevent pilferage of seeds by animals tree species in the region (Jenkins and Parker 1998), while still allowing mast to fall into the trap. Traps and to ensure that both oak sections were represented. were placed midway between the trunk and canopy Individual trees of reproductive age were chosen based edge underneath a limb. For trees adjacent to proposed on their location relative to future harvests. harvest areas, one mast trap was placed on the side of the tree facing the harvest and one on the opposite The number and location of selected trees depended side. For all other trees, one mast trap was oriented to on the assigned management treatment of the unit.

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